1) derived from adding mc onto every word.
2) refers to a boyfriend who has weird habits, crab claws your chest or wont have sex with you even though he has no chance of getting laid.
3) someone who has bad acne and sort of looks like a gremlin because of it

1) The roadkill we just passed is mcnasty.
2) Mcnasty is coming over but we wont DO anything.
3) I got put on this cream because my face was mcnasty.